The present invention relates to the field of vacuum cleaner fans. In conventional vacuum cleaners, a fan drives dirt laden air into a filter bag. There are two common vacuum cleaner configurations. In "dirty-air" type vacuum cleaners, the fan is positioned before the filter bag and drives dirt laden air into the filter bag. In "clean air" type vacuum cleaners, the fan is positioned after the filter bag and sucks clean air out of the filter bag.
FIGS. 1, 2A and 2B show a conventional dirty-air vacuum cleaner 10. A fan 12 drives air from a floor nozzle 14 to a filter bag via a fill tube 18. Dirt removed from the floor by the airflow is thus filtered out and deposited into the filter bag 16.
The fan 12 comprises a motor 20, a housing 22, and an impeller 24. The motor 20 is connected to the back of the housing 22 and rotates the impeller 24 via a shaft 26. The resulting centrifugal force draws air into an inlet 28 and out through an outlet 30. The housing comprises a back wall 32, a substantially flat front wall 34, a volute 36 (scroll-shaped side wall), and a cutoff 38. As air is swept around the housing 22 by the impeller 24, the air fills the continually growing gap between the impeller 24 and the volute 36 until it is redirected to the outlet 30 by the cutoff 38.
FIGS. 3A and 3B are detailed views of an impeller 24 of the type commonly used in dirty-air vacuum cleaners. The impeller 24 comprises a hub 42 supporting a backplate 44 which supports multiple blades 46. The hub 42 has a bore 48 for mounting onto the motor shaft 26. Each blade 46 has a leading edge 50, a top edge 52, and a trailing edge 54. The entire impeller 24 is usually molded from plastic.
Conventional impellers for dirty-air fans typically include a number of design features which are engineered into the impeller design to improve air performance (i.e. performance in terms of suction and airflow) and reduce fan noise. The empty area between hub 42 and blades 46 is called the "eye" 49 and provides more space for air entering the inlet 28. The leading edge 50 is sloped upward to streamline airflow where it first encounters the blade 46. The backplate 44 is curved, as shown, to soften the airflow's right angle turn when it first hits the backplate from the inlet 28. The blades 46 are generally not aligned radially but are rather backswept relative to the rotational direction and are typically curved.
In conventional impellers for dirty-air fans, the top edge 52 of the blade 46 is substantially parallel to the front wall 34. So if the front wall 34 is flat and perpendicular to the shaft 26, as is typical, the top edge 52 is also perpendicular to the shaft 26. Similarly, the trailing edge 54 is substantially parallel to the volute 36. So if the volute is generally parallel to the shaft 26, as is typical, the trailing edge 54 is also parallel to the motor shaft. Hence, if the front wall 34 is perpendicular to the volute 36, as is typical, then top edge 52 is perpendicular to trailing edge 54.
In order to establish the airflow required for removing dirt, the impeller must rotate at high speed, typically 10,000-20,000 RPM. The strong centrifugal force acting on the impeller's mass applies several stresses to the impeller: the curved backplate is stressed, causing it to straighten out and pull away from the blades; the blade curvature is stressed to horizontally straighten out; and the backswept blades are stressed to tip over onto the backplate. The repeated on-off application of these stresses can produce damage such as: stress cracks in the backplate; weakening of the joint between the blade and backplate; gradual deformation of the blade shape; and fatigue the material. All this stress damage degrades air performance and impeller durability, in addition to increasing the noise level.
Besides stress-related damage, there is also impact damage. The blades can become chipped, usually at their trailing edge 54, by small hard objects picked up by the vacuum cleaner which hit the impeller with a violent impact.
Dirty-air fans tend to be loud due to air turbulence within the housing. Also, the repetitive passing of the trailing edges 54 past the cutoff 38 produces a siren effect. Within the fan housing, the cutoff 38 represents the region of smallest clearance between the volute 36 and the impeller 24. As each blade passes the cutoff 38, a pressure pulse is generated which produces a sound. The pitch of the sound is at a frequency corresponding to the rate of blade passage past the cutoff. This frequency is called the "blade-passing frequency."
Applicant has observed several performance-related factors in connection with a standard impeller, i.e. impeller no. MO-118978, used in many Kirby vacuum cleaners. The dimensions of this impeller type are as follows: there are 11 blades standing vertical from a curved backplate; the backplate's outer diameter is 121 mm; the blade's top edge is within a horizontal plane (i.e. taper of 0 degrees), and is 21 mm high (measured from the backplate's outer edge); the blades' leading edges intersect the backplate at 23 mm from the hub center, and are tapered at 45 degrees from vertical; the blades' trailing edges are vertical (i.e. zero taper) and intersect the backplate essentially at the backplate's outer edge; the backsweep of the curved blade, measured relative to radial, is 45 degrees at the leading edge and 37 degrees at the trailing edge.
The impeller resides within a standard Kirby G4 model fan housing having dimensions as follows: the front face is horizontal and is 28 mm from the back face; the inlet diameter is 50 mm; the clearance between the blades' top edges and the housing's front face is uniformly 4 mm; the volute is vertical in one dimension and has a radius that increases from 63 mm on one side of the cutoff to 110 mm just after the cutoff; the clearance between the blade's trailing edge and the volute is 3 mm at the cutoff and increases by about 7.4 mm for each 1/4 rotation away from the cutoff.
The standard fan, having the aforementioned dimensions, produces maximum suction of 28 inches of water, maximum airflow of 110 CFM, produces 94 dBA noise pressure level (measured from 3 feet away) when the cleaner is used in a 15,000 RPM "shampooer mode" and 80 dBA when normally 12,000 RPM while vacuuming plush carpet. In a standard "shrapnel impact" test (where nuts, bolts, pennies, washers and bobby pins are sucked into the cleaner's suction hose), the standard impeller typically tends to crack after 400 impacts on average.